Edible oils have been refined from ancient times, but with a few exceptions these efforts have centered on animal fats and oils. Serious efforts to refine vegetable oils, such as soybean, cottonseed, or palm oil, have been made in this century, and products of this type are now freely available. An early process for treating raw edible oils of this type is disclosed in U.S. Pat. No. 2,441,923 of Francis M. Sullivan and utilizes heat treatment followed by deodorizing.
More complex processes for treating raw edible glyceride oils have been developed more recently as described in U.S. Pat. No. 4,150,045 to Rabindra K. Sinha entitled MgO IMPREGNATED ACTIVATED CARBON AND ITS USE IN AN IMPROVED VEGETABLE OIL REFINING PROCESS. This patent describes the refining processes of the prior art as applied to raw vegetable oil as consisting of degumming, alkali neutralization, water washing, bleaching, and deodorizing performed in that order.
The contaminants of raw glyceride oil and the contaminants of used cooking oil are generally different due to introduction into the cooking oil of food juices and particles and the effects of oxidation on the oil, but there are some common contaminants, such as fatty acids. In the refining of raw glyceride oil, the fatty acids combine with metallic ions from the processing equipment to form soap, and soap also is produced in cooking oil by the cooking process as a result of the combination of fatty acids and metallic ions from the cooker. An increase in the concentration of free fatty acids in glyceride oil, or the production of significant amounts of soap, is considered to be a precursor of rancidity in the oil.
The art has several methods of treating used cooking oil, all of which have the effect of reducing the production of soap in used cooking oil. U.S. Pat. No. 4,764,384 of John Gyann entitled METHOD OF FILTERING SPENT COOKING OIL adsorbs free fatty acids by treating used cooking oil with a composition of silicates including a hydrated amorphous silica gel, thereby reducing the combination of free fatty acids and metallic ions and the resulting soap. Another approach to controlling the concentration of free fatty acids by adsorption is described in U.S. Pat. No. 4,235,795 of Cohen, and this approach admixes pumicite with used cooking oil as an adsorbent for free fatty acids. A second method of treating used cooking oil is described in U.S. Pat. No. 4,330,564 of Friedman in which a chelating agent is admixed with the cooking oil to tie up the metal ions and prevent the combination of free fatty acids and metal ions and the resulting production of soap. A third method of treating used cooking oil is described in U.S. Pat. No. 3,231,390 of Hoover in which used cooking oil is treated with an adsorbent consisting of an alkaline earth metal carbonate or an alkaline earth metal oxide, and the adsorbent is removed from the oil by filtration. It is believed that the process of Hoover is a saponification process which removes the metallic ions as soap in the filtration step.
While removal of soap from spent cooking oil is an important consideration in restoring the usefulness of the oil and increasing the useful life of the oil in the cooking process, there are other contaminants in the used cooking oil which should preferably be removed. By practicing the present invention, phospholipids, peroxides, crumbs and food fragments, and other impurities, as well as fatty acids and soap, are removed from the used cooking oil.